1. Field of the Invention
The present invention generally relates to a solid fuel combustion system with improved combustion and aesthetics and, more particularly, to a solid fuel combustion device with a limited travel air supply intended to, amongst other things, simplify operation and reduce emissions of air borne pollutants.
2. Background Description
In the mid 1980's growing concern over ambient air quality caused regulators to focus on wood burning appliances as sources of significant amounts of particulate matter and other pollutants which posed a threat to human health. Hardware commonly known as "wood heaters" were the subject of a federal new source performance standard in 1988. This standard required the certification of all new wood heaters sold in the United States and was intended to cover only those products which were capable of burning at low air/fuel mixtures, a condition which can lead to high emissions of particulate matter (PM), carbon monoxide (CO) and other organic pollutants.
Wood burning appliances falling within the Environmental Protection Agency (EPA) definition of a "wood heater" must be certified as clean burning by meeting specified emissions criteria when tested in a laboratory using standardized test methods. The standard specifically defines wood heaters based on performance characteristics, their intended use and size. Site built masonry fireplaces, cookstoves, boilers and central heaters, and masonry heaters were exempt from this federal regulation. Fireplaces are not automatically exempt from regulation but gain exemption through application of EPA Method 28A (see 40 C.F.R. .sctn.60 (1988)) which is a standardized test method determining minimum burn rate and air-to-fuel ratio. Using this test method, any device exhibiting an average bun rate of higher than 5 kg/hr or an air-to-fuel ratio of higher than 35 to 1 is determined not to be a wood heater and is therefore exempt from federal regulation.
The EPA Method 28A is accepted as a reference method for determining specific operational characteristics of a wood burning appliance. Procedures for determining the minimum burn rate and the average air-to-fuel ratio are specified. The following discussion makes reference to specific burn rates and air-to-fuel ratios and unless otherwise specified, EPA Method 28A is the reference method for determining the specified values. Similarly, the term "full load" in the following discussions refers to the fuel load specified by EPA Method 28A and is considered representative of the largest fuel load likely to be encountered with use of the wood heater.
Numerous studies of emissions from EPA certified wood burning stoves have shown that field performance can vary widely depending on, among other things, fuel quality, mechanical degradation and operator actions. Poor or unpredictable performance, in effect, circumvents the intent of mandating EPA certified wood heaters since emissions of pollutants are not controlled as desired. While the factors of fuel quality and mechanical degradation can be remedied, operator performance is very difficult to control. Proper operation of air controls and bypass dampers is critical to ensuring proper emissions reduction in current certified stove models and the factors of installation, fuel properties, heating needs and even weather will require different operation from day to day or from household to household. With these factors in mind the actions or inactions of the operator when using the stove controls can be critical to effective stove performance.
Further and more specifically, current technology wood stoves have operator controls which if used improperly can cause poor performance. Wood stoves may include catalytic converters or tuned secondary air systems which serve to reduce emissions by enhancing combustion efficiency or combusting the pollutants within the effluent stream prior to entering the chimney or venting system. These systems require operator knowledge as the stoves and/or catalytic combustors must be sufficiently heated in order to be effective in emissions reduction. In the case of catalytic stoves, actuation of a bypass diverts the flow of combustion products through the catalytic combustor. If the bypass damper does not get actuated or the catalyst itself is not sufficiently heated and the stove is banked soon after fuel loading, the catalyst might not get lit and no emissions reductions are achieved. Similarly, there is opportunity for non-catalytic stoves to be banked too soon, even when using proper fuel, since preheating of the secondary air system is necessary to combust volatile organic materials evolved from the wood. Once the stove is banked and the air-to-fuel ratio (mass of air divided by mass of fuel) is overly reduced in these devices, flaming may cease and the wood stove might enter a smoldering phase which can last for the entirety of a fuel charge. These scenarios are supported in the field data and are considered undesirable.
Further, with the continuing concern over wood smoke, some localities, particularly in the Western region of the United States have widened the scope of their regulations to restrict or ban residential solid fuel burning devices which are not federally regulated. These include what are commonly known as fireplaces and masonry heaters. While these devices have served a need and have been popular in homes for centuries, some local regulations allow only EPA certified devices to be installed. Since masonry heaters and fireplaces are not affected facilities under federal law, no means of certifying their performance exists and the devices cannot be installed, or in some cases even used, in these localities. EPA certified wood stoves using current technology emissions control systems attempt to fill the need of fireplace customers however, the expense of added operator controls, pollution reduction equipment and, in general, heavier airtight welded construction make the cost of these devices higher than is desirable. Also, the complexity of user controls is higher than it need be for primarily decorative appliances, possibly resulting in operator error and less than desirable performance.
Fireplaces typically have little if any combustion air control and are intended primarily as decorative devices, although some models can be used as supplemental heaters as well. Inefficiencies of fireplaces result from high fuel burning rates and high air-to-fuel ratios as compared to wood stoves which are primarily intended for heating. Combustion efficiency can be relatively good due to the abundance of air and the presence of flaming; however, too much air can have a quenching effect which inhibits efficient combustion. Even if the combustion efficiency is relatively high (as indicated by low pollutants per unit mass of fuel), the uncontrolled high fuel burning rate can result in high emission rates (mass of pollutant per unit time), which is the measure of emissions of primary concern to air pollution regulators.
Currently, a great variety of wood burning systems have been described and demonstrated in the prior art. Indeed, "fireplaces" and "woodstoves" have been in existence for hundreds of years but operationally, efficiency and pollution concerns still exist which are not adequately addressed with the current state of the art. Wood burning appliances may be classified as "open" or "closed" combustion devices. The term "open" refers to un-controlled; un-regulated or fuel-lean operation as in "fireplaces", while the term "closed" implies controlled, regulated or fuel rich combustion as in "woodstoves". Un-regulated wood burning systems have low heating efficiency due to high flow rates of combustion or cooling air while regulated systems exhibit low combustion efficiency as a result of operating in a fuel rich range which, in turn, results in incomplete combustion of the organic components of the fuel and higher emissions.
Prior art systems have sought to improve the performance of either controlled or uncontrolled devices in a wide variety of ways. In the case of fuel rich devices (wood stoves), a variety of pollution control technology intended to enhance combustion efficiency when a device is operating in a fuel rich condition have been described in the art. These include the use of complex secondary combustion air introduction systems as in U.S. Pat. No. 4,766,876 to Henry, et al. or the use of catalytic converters as in U.S. Pat. No. 4,330,503 to Allaire, et al.
Many examples of improvements to uncontrolled, lean-burning combustion chambers have also been used and described for over one hundred years. While combustion efficiency is quite good relative to fuel-rich devices, low overall efficiency can result if the high sensible heat loss resulting from high air flow and relatively high fuel burning rates is not recovered. Prior art systems describe several heat recovery system which have been successful to varying degrees. These include the use of heat transfer chambers, long and tortuous flow paths and thermal mass storage, just to name a few. However, the known prior art devices are not operable at an average fuel consumption rate below 5 kg/hr when tested using accepted industry standards and in fact, in many instances, are intended to operate at much higher burn rates. This results in less than desirable efficiency for the reasons stated above. Significant overall efficiency improvement is made by reducing the combustion air flow and consequent burn rate.
In further examples, U.S. Pat. No. 4,368,722 to Lynch describes a device which, among other things, seeks to maintain a combustion zone within a fuel charge by novel introduction of controlled amounts of combustion air. The flow path and geometry of this air introduction are intended to help produce a lean combustion "zone" whereby complete combustion can occur. However, as in all known prior art relating to fuel rich wood burning devices, the Lynch system includes an adjustable air introduction system for "providing exactly the amount of air desired for proper combustion", but the proper amount of air is not specified. In fact, the combustion air can be over-dampened since the inlet controlling damper may be closed enough to allow the system to operate in a fuel-rich, non-flaming condition. Considering the teachings of the Lynch system, a stove capable of being throttled too much is capable of non-flaming or smoldering combustion which would require a "clean-up" technology to handle the resultant emissions. If the clean-up technology is ineffective (do to inefficiency, degradation or improper use) no emissions reduction is achieved.
In U.S. Pat. No. 20,667 to Savage, a heat stove with air introduction is described as a "self-regulating" air supply. Savage, however, is related only to the specific means and geometry of air introduction, and the range of operation is not specified.
What is needed in the art is a wood burning heater which burns standard firewood and ensures proper emissions performance independent of operator actions and minimizes or eliminates the requirements of proper control actions to achieve reduced emissions. A further need is a simply operated wood burning heater which effectively reduces emissions of pollutants while providing the decorative function of a fireplace.